The present invention relates to brazing alloys and, more particularly, to brazing alloys for brazing tungsten carbide-cobalt composites to titanium alloys.
Tungsten carbide-cobalt materials (herein WC—Co) often are used to make various parts and components for aircraft engine applications due to the high mechanical strength, hardness, corrosion resistance and wear resistance of WC—Co. For example, wear resistant carboloy pads used in aircraft engines typically are constructed from (90-98 wt %) WC and (2-10 wt %) Co mixtures. The WC—Co carboloy pads typically are brazed to fan and compressor blade midspan shrouds for wear applications in aircraft engines. These blades typically are made of Ti 6Al-4V and/or Ti 8Al-1V-1Mo alloys with beta transus temperatures at or slightly above 1800° F.
In the prior art, titanium/nickel/copper braze alloys (herein TiNiCu), such as Ti-15Ni-15Cu, have been used to braze carboloy pads to titanium alloy blade midspan shrouds. TiNiCu braze foils have also been used for brazing WC—Co to titanium alloys since TiNiCu is the main braze alloy for brazing of titanium alloys with good strength and ductility. However, TiNiCu alloys have presented various impact failure problems when used in applications involving the brazing of WC—Co to titanium alloys, including chipping and fracturing at the braze joint when the brazed pads are subjected to an impact force (e.g., collision with a bird, an adjacent blade or various debris).
It has been found that the braze impact failures may be attributed to the low ductility brittle braze joints formed when brazing WC—Co to titanium alloys using TiNiCu brazing alloys. In particular, it has been found that tungsten and cobalt from the carboloy pad dissolves into the braze joint when the TiNiCu brazing material is in the molten state, thereby forming a low ductility, high hardness (e.g., about 1200 KHN) W—Co—Ti—Ni—Cu alloy braze interface. The braze interface exhibits cracking at impact energies as low as 0.30 joules and the carboloy pad is liberated from the substrate at the brittle braze interface at an impact energy of 0.60 joules.
Thus, TiNiCu braze alloys that have been successfully used for brazing of titanium to titanium alloys cannot be used for brazing of WC—Co to titanium alloys where impact resistance is required.
Industrially available braze alloys have been unable to meet the combined demands of low braze temperatures (i.e., below 1800° F.), high ductility and low cost necessary for aircraft engine applications. For example, Nioro (Au 82% and Ni 18%) and Nicoro80 (Au 81.5%, Cu 16.5% and Ni 2%) are heavy in gold and light in copper and therefore are expensive and have poor wetting properties and ductility. Furthermore, alloys incorporating Au 35%, Cu 62% and Ni 3% have liquidus temperatures at or above 1886° F., which is not applicable for brazing WC—Co to titanium alloys.
Accordingly, there is a need for ductile, impact resistant brazing alloys with brazing temperatures below the beta transus temperature of the substrate titanium alloy. In particular, there is a need for brazing alloys for brazing WC—Co materials to titanium alloys without forming a brittle braze interface.